1. Field of the Invention
The present invention relates to the field of bulk acoustic resonators.
2. Prior Art
Bulk acoustic wave (BAW) resonators are used to provide RF filtering for wireless application, and may also be used in reference oscillators. However, the cost and performance of a BAW resonator is quite dependent on the electrodes used. The problem for BAW resonator electrodes is to combine a set of requirements that can appear at times non-compatible. A BAW resonator electrode should have:                low resistance to limit insertion losses        high acoustic impedance to confine the energy within the active stack of the device and achieve good keff2        allow for a good growth of highly oriented polycrystalline AlN in the case of the bottom electrode        be fully integrable, that is, allow for a realistic manufacturability with good etch selectivity in the process.        
The resulting stack making up the BAW resonator, including these electrodes, needs to show a Thickness Extensional mode branch in the dispersion curves plot (whose domain is wave number in abscissa, frequency in ordinate) favorable for spurious mode control (usually targeted with a positive slope, but qualitatively flat).
U.S. Pat. No. 6,291,931 discloses the use of a layer of W and a layer of Al for BAW resonator electrodes. However it has been found that tetra-methyl-ammonium-hydroxide (TMAH), particularly heated and used in a wet immersion etch, is a superior etching process for AlN. However in cases where the W layer is porous, which it may be, the TMAH etches the Al layer, which in the case of the bottom electrode, results in failure of the device.